If caching is turned on (default=on), any write will only be acknowledged back to the host after it has been succesfully stored in DRAM memory of two separate physical SANsymphony nodes. Based on de-staging algorithms each of the nodes eventually copies the written data that is kept in DRAM to the persistent disk layer. Because DRAM outperforms both flash and spinning disks the applications experience much faster write behavior.
Per default, the limit of dirty-write-data allowed per Virtual Disk is 128MB. This limit could be adjusted, but there has never been a reason to do so in the real world. Individual Virtual Disks can be configured to act in write-through mode, which means that the dirty-write-data limit is set to 0MB so effectively the data is directly written to the persistent disk layer.
DataCore recommends that all servers running SANsymphony software are UPS protected to avoid data loss through unplanned power outages. Whenever a power loss is detected, the UPS automatically signals this to the SANsymphony node and write behavior is switched from write-back to write-through mode for all Virtual Disks. As soon as the UPS signals that power has been restored, the write behavior is switched to write-back again.
DataCore, SSY [SDS]/10.0 PSP10, x86
Persistent Write Buffer, Reads/Writes, Data Availability